Acoustical Treatment for Indoor Areas 99typically encountered with acoustical materials that are
spray or trowel applied. The material is first applied to a
solid backing board and then tested by placing the
treated boards over the predefined test area in the
chamber. Type E mounting is the standard method
employed for absorbers such as acoustical ceiling tiles.
This mounting includes a sealed air space of a defined
depth behind the absorbers to mimic the real-world
installation of acoustical ceiling tiles with an air plenum
above. The depth is defined in millimeters and is
denoted as a suffix. For example, a test of acoustical
ceiling tiles in an E400 mounting means that the tiles
were tested over sealed air space that was 400 mm
(16 in) deep.
It should be noted that Type A mounting for
board-type wall and ceiling absorbers is so often used as
the default method that any mention of mounting method
is often carelessly omitted in manufacturer literature.
Regardless, it is important to verify the mounting
method used when evaluating acoustical performance
data. If there is any uncertainty, a complete, independent
laboratory test report should be requested and evaluated.
Details of the mounting method must be included in the
lab report to fulfill the requirements of the test standards.
ASTM C423 is generally used in certified North
American laboratories; ISO 354 is generally the adopted
standard in European countries. The methods are very
similar, but there are some noteworthy differences that
can yield different testing results. A main difference that
is a frequent subject of criticism is the different
minimum sample sizes. The minimum area of material
when testing board-type materials in accordance with
ASTM C423 is 5.6 m^2 (60 ft^2 )^5 (the recommended test
area is 6.7 m^2 [72 ft^2 ]) and that of ISO 354 is 10 m^2
(107.6 ft^2 )^6. In general, this difference in sample size can
result in a material having slightly lower absorption
coefficients when tested in accordance with ISO 354
relative to the same material tested in accordance with
ASTM C423. The ISO method is generally regarded as a
more realistic approach when the test results are being
applied to spaces that are larger than the test chamber, as
is often the case. Nonetheless, ASTM test results have
been widely and successfully used in architectural
acoustic room design applications for many decades.
The reverberation chamber methods can also be
applied to discrete absorbers, such as auditorium seating,
highway barriers, office partitions, and even people. The
main difference between testing the discrete absorbers
and testing panel-type absorbers is how the results are
reported. If a material occupies a commensurable area of
a test chamber surface, absorption coefficients can be
calculated. By contrast, the results of a test of some
number of discrete absorbers are generally reported in
sabins/unit. (Sometimes referred to as Type J mounting
in the literature, provided the test met the standard
requirements for that mounting). For example, the
absorption of acoustical baffles—the type that might be
hung from a factory or gymnasium ceiling—is typically
reported in sabins/baffle.
When calculating absorption coefficients for
board-type absorbers, the number of sabins in each
frequency band is divided by the surface area of the test
chamber covered by the sample material. The resulting
quantity is the Sabine absorption coefficient, abbreviated
DSAB. The vast majority of absorption coefficients
reported in the literature is Sabine absorption coeffi-
cients. Since the material is tested in a reverberant space,
the Sabine absorption coefficients are useful for prede-
termining the acoustical properties of a space, provided
that the product is intended for use in a similarly rever-
berant space (i.e., a space where sound can be consid-
ered to be impinging equally on a surface from all angles
of incidence).
The frequency range of reverberation chamber
measurements is limited. At low frequencies, modal
effects can dominate the test chamber, thus making accu-
rate measurements of sound decay difficult. At high
frequencies, the chambers are large enough that the
absorption of air will start to affect the measurement
results. Therefore, the frequency range for a reverbera-
tion chamber test is typically limited to the octave
bands between 100 and 5000 Hz. This is sufficient for
most materials and applications as it spans a full six
octaves over what is commonly referred to as the speech
range of frequencies—i.e., the range of frequencies that
are important to address design issues related to speech
communication.
When acoustical treatments are specifically designed
to absorb low frequencies, the reverberation chamber
method can fall short. However, D’Antonio has imple-
mented a special application of the ASTM C423 method
that utilizes fixed microphone positions (vs, the more
typical rotating microphone) that measure the decay of
the actual modal frequencies of the room. Using this
method, D’Antonio has been able to measure low
frequency absorption down to the 63 Hz octave band.8,9
The impedance tube method (discussed below) can also
be used to measure low frequency absorption, but a large
tube with heavy walls (such as poured concrete) is
required.(^1) » 3